One of the most important quality requirements of all paper and board grades is uniformity of the structure both on the micro scale and on the macro scale. The structure of paper, in particular of printing paper, must also be symmetric. The desired printing properties required from printing paper mean good smoothness, evenness, and certain absorption properties of both faces. The properties of paper, in particular the symmetry of density, are affected to a considerable extent by the operation of the press section of the paper machine, which operation also has a decisive significance for the uniformity of the profiles of the paper in the cross direction and in the machine direction.
Increased running speeds of paper machines create new problems to be solved, which problems are mostly related to the runnability of the machine. Currently, running speeds of up to about 1500 meters per minute are employed. At these speeds, so-called closed press sections, which comprise a compact combination of press rolls positioned around a smooth-faced center roll, usually operate satisfactorily. As examples of such compact press sections, reference is made to the current assignee's Sym-Press II.TM. and Sym-Press O.TM. press sections.
From the point of view of energy economy, dewatering taking place by pressing is preferable to dewatering taking place by evaporation. For this reason, attempts should be made to remove a maximum amount of water out of the paper web by pressing in order that the proportion of water to be removed out of the paper web by evaporation may be made as small as possible.
Increased running speeds of paper machines, however, create new, so far unsolved problems expressly for the dewatering taking place by pressing, because the press impulse applied when dewatering by pressing cannot be increased sufficiently by the means known from the prior art, above all because at high web running speeds, the nip times remain inadequately short and, on the other hand, the peak pressure of pressing cannot be increased beyond a certain limit without destruction of the structure of the web.
With increasing running speeds of paper machines, the problems of runnability of a paper machine are also manifested with further emphasis, because a web with a high water content and low strength does not withstand an excessively high and sudden impulse of compression pressure or the dynamic forces produced by high speeds, but web breaks and other operational disturbances arise which result in standstills and in considerable economic losses.
Further problems which are manifested with increased emphasis at high web running speeds of paper machines and for which satisfactory solutions have not been found so far, at least not for all of the problems, include problems of quality related to the requirements of uniformity of the machine-direction and cross-direction property profiles of the paper web. The uniformity of the web that is being produced also affects the runnability of the whole paper machine, and it is also an important quality factor in finished paper, which factor is emphasized with respect to copying and printing papers with higher speeds of copying and printing machines and with increased requirements concerning the uniformity of the printing quality.
The machine-direction property profiles of the paper produced are also affected significantly by oscillations in the press sections, and the variations of properties in the cross direction are affected by the cross-direction profiles of the nip pressures in the press nips. These profile problems tend to be increased considerably with increasing running speeds of the machine.
In recent years, speeds even as high as about 40 meters per second (about 2400 meters per minute) have been contemplated as running speeds of paper machines. The achievement of speeds as high as this, in particular in wide machines, results in ever more difficult problems to be solved, of which problems the most important ones are the runnability of the machine and an adequate dewatering capacity at a high web speed.
With respect to the patent literature most closely related to and connected with the present invention, reference is made to the following publications:
______________________________________ Finnish Patent Nos. 81,854 (corresponding to U.S. Pat. No. 4,526,655), 82,500, 85,044 (corresponding to U.S. Pat. No. 4,861,430), and 93,563 (corres- ponding to International Publi- cation No. WO 88/08051); Finnish Patent Application Nos. 842115 (corresponding to U.S. Pat. No. 4,931,143), 950451 (filed February 2, 1995), and 951934 (filed April 24, 1995); U.S. Pat. Nos. 4,483,745, 4,561,939, 4,648,942, 4,915,790, 4,943,351, 4,988,410, 5,087,325, 5,169,501, and 5,368,697; European Patent Publication Nos. 0 159 280 B1, 0 344 088 A2, and 0 496 965 B1; German Patent Publication Nos. 36 04 522 A1 (corresponding to Finnish Patent No. 82,500), 37 42 848 A1 (corresponding to U.S. Pat. No. 4,915,790), 42 27 000 A1, 44 02 629 A1; International Publication Nos. WO 88/08051 (corresponding to Finnish Patent No. 93,563) and 95/16821; and Canadian Patent Application No. 2,034,829. ______________________________________
Further, reference is made to the constructions illustrated in the accompanying FIGS. 8A and 8B, mainly included in the prior art and available at least to the current assignee.
In the prior art press sections, in particular in press sections meant for producing printing papers, the last press nip is generally a single-felt nip, and the transfer of the web after the last nip has taken place so that the web is separated from the press felt of the last nip and is transferred on a smooth face of the press roll, from which roll face the web is separated and transferred as an open and unsupported draw onto the drying wire. The free draw is advantageous in view of the difference in speed needed in order to maintain a web tension, but the open draw causes a considerable risk of web breaks, in particular at higher speeds, so that free draws can, as a rule, not be employed at speeds higher than about 1700 meters per minute. The use of a single-felt last nip may also cause the drawback that the web becomes asymmetric in respect of the smoothness properties of its opposite faces because the face of the web that is pressed against the smooth press roll in the last nip receives a higher smoothness than the opposite web face, which was placed against the water-receiving felt. The unequalsided draining of water taking place in the last nip can also distort the distribution of fillers and fines in the web. Thus, the single-felt last press nip in the prior art press sections tends to produce a poor symmetry of roughness in particular with fine paper and with LWC and MWC base paper.
This problem is exacerbated when the press impulse is high, as is the case in an extended-nip press in the last press position. For example, with MWC base paper, in the current assignee's test paper machine, a non-calendered ratio of 0.52 was obtained for Bendtsen roughness of top side to bottom side when the press load was, in a "Sym-Belt S".TM. press, about 800 kN per meter, when the length of the press shoe was about 152 mm, and when the smooth press roll was placed in the upper position in the single-felt press nip. The extensive asymmetry of roughness constitutes a limitation for the magnitude of the press loading, for the dry solids content that can be attained, and for the wet strength.
It is known from the prior art to use what are called equalizing presses in connection with various press sections, also extended-nip press sections, by means of which equalizing presses attempts are made to equalize the above asymmetry of roughness. With respect to these prior art equalizing presses, reference is made, for example, to the current assignee's Finnish Patent No. 64,823 (corresponding to U.S. Pat. No. 4,566,946), to published German Patent Application No. 43 21 406 A1 in the name of Messrs. J. M. Voith GmbH (corresponding to U.S. Pat. No. 5,468,349), and to German Utility Model No. G 92 06 340.3 in the name of Messrs. Sulzer-Escher Wyss GmbH. By means of the equalizing presses known from the above publications, it has, however, not been possible to provide satisfactory solutions for the problems related to asymmetry of roughness, in particular not in connection with supported transfer and closed draw of the web.
The prior art includes a number of arrangements in which the transfer of the web from one fabric onto another fabric, or further along the web formation path, or ensuring that the web follows exactly the press fabric that is supposed to carry the web further has been accomplished by means of a transfer suction roll or some other suction device. However, it is a drawback of the use of these suction devices that they cause rewetting of the web because of their suction effect. This rewetting is particularly detrimental in particular after the last nip in the press section, in which nip the web is already relatively dry and capable of absorption and, thus, particularly susceptible to rewetting. The risk of rewetting has imposed considerable restrictions for the use of transfer suction devices and for the application of vacuums sufficiently high in view of the transfer of the web.
In the press section of a paper machine, for the transfer of the web, various transfer belts are also employed, which do not substantially receive water and which are substantially impenetrable, the operation of these transfer belts being based mainly on their surface properties, because a suction effect that promotes or guarantees the transfer of the web cannot be applied to the web through such belts.